Self-threading polymeric fastener

Abstract

A self-threading polymeric fastener and blank therefor which can be used, for example, to support a wall plate over an electrical outlet or switch. The fastener blank comprises three portions, all integrally molded together, including a slotted head, a substantially cylindrical shank and a tapered tip. A helical thread is formed on the shank and tip by deforming the outer surfaces thereof without removing any significant amount of polymeric material therefrom. This is accomplished by rotating the tip and shank in a helically threaded bore, such as that provided in an outlet or switch. The polymeric fastener blank can be integrally molded with a polymeric wall plate to make their colors identical.

What is claimed is: 
     
       1. A fastener assembly comprising: a member having a threaded bore therein;   a plate having an aperture therethrough; and   a self-threading polymeric screw comprising a head having a transverse slot for receiving a rotatable screw driving mechanism,   a shank having first and second ends, the first end extending integrally from said head, said shank being substantially cylindrical,   a tip extending integrally from the second end of said shank, said tip tapering inwardly from said shank, and   a helical thread permanently cold formed on said shank by deforming the outer surface of said shank without removing any significant amount of polymeric material therefrom by rotating said screw in said threaded bore,     said shank being received in the aperture in the plate,   said helical thread formed on said shank being threadedly engaged with the threads in the threaded bore,   said plate and said screw being comprised of the same polyamide resin,   said helical thread having rounded crests.   
     
     
       2. A fastener assembly accoring to claim 1 wherein the plate is a wall plate, and   the member is an electrical outlet or switch.   
     
     
       3. A fastener assembly according to claim 1 wherein said helical threads of said screw shank have first diameters at said thread crests;   said threaded bore having major diameters between the roots of said screw threads and minor diameters between the crests of said screw threads, the major diameters being greater than said minor diameters;   said first diameters being between the minor diameters and said major diameters.   
     
     
       4. In a fastening assembly including a wall plate, an electrical wiring device and a rotatable polymeric fastener for mounting the wall plate on the electrical wiring device, said device having a bore with an internal helical thread with respective root and crest portions and a thread groove of V-shaped cross section between adjacent crest portions, the improvement wherein the fastener comprises: an elongated shank formed with an external helical thread having respective root and crest portions, said crest portions being rounded,   said root portions of said fastener having a diameter substantially equal to the diameter of said crest portions of said internal thread of the device for making close threaded engagement therewith, and   said crest portions of said fastener being shaped to occupy less than the full cross-sectional area of said thread groove,   whereby said crest portions of said fastener slip over said crest portions of the device upon over torquing rotation of said fastener,   said fastener being comprised of the same material as said wall plate.